High nitrogen levels measured in the West Fork of the Gallatin River

Nitrogen levels measured in the West Fork of the Gallatin River during summer 2018 were some of the highest ever recorded, which may have contributed to increased algae in the Gallatin River, according to the Gallatin River Task Force.

This summer, unprecedented levels of bright green algae were observed covering the bottom of the Gallatin River between Beaver Creek and Portal Creek and on several tributaries, including the West Fork, South Fork of the West Fork, and Taylor Fork. Algae blooms are a concern because they alter aquatic insect habitat and can be a nuisance when swimming or fishing. Severe algae blooms can lead to decreased dissolved oxygen levels in stream water, which harm fish and stream insects.

The Gallatin River Task Force is a nonprofit organization that monitors the health of the watershed and collects regular data at 21 sites on the Gallatin River and the streams that feed it to track baseline river conditions and understand events, like elevated algae growth. Nitrogen analyses were among the first test results the organization received from their summer monitoring.

“These results suggest nitrogen levels were likely one of the factors that contributed to the increase in algae in the Gallatin River downstream of the West Fork, but we are awaiting several test results and subsequent data analyses that will give us further insight into the cause of the bloom,” said Kristin Gardner, executive director of the task force.

Environmental conditions, that include levels of nutrients, like nitrogen and phosphorous; water temperature; streamflow; water clarity; and sunlight, drive algae growth. Researchers must evaluate all these factors together to determine the reason for algae blooms.

In the past, excess nitrogen has contributed to nuisance algae growth on three major streams in the Big Sky area: West Fork of the Gallatin River, Middle Fork of the West Fork, and South Fork of the West Fork. Potential sources of excess nitrogen to local streams include over-irrigating golf courses with treated wastewater, improperly maintaining septic systems, applying too much fertilizer to outdoor landscapes, and concentrated areas of pet and horse waste.

The task force measured the highest nitrogen levels of the summer in the West Fork above the South Fork confluence: 0.42 milligrams/liter for total nitrogen and 0.31 mg/L for nitrate. In contrast, the state standard for total nitrogen is 0.3 mg/L. The state standard for nitrate was removed by the Montana Department of Environmental Quality in 2013; however, the task force still uses the previous standard of 0.1 mg/L as a trigger value of elevated nitrate in local streams.

The only historical nitrate measurements similar to the summer 2018 values were collected at two sites on the West Fork downstream of the Big Sky Community Park in August 2008. At that time, water quality assessments documented excess algae in the South Fork and West Fork that extended into the Gallatin downstream of the West Fork confluence. Although algae levels were not measured in the Gallatin in 2008, visual observations by frequent river users suggest that they were not as high as 2018.

The task force expects to receive additional results in November and will collaborate with experts from the Montana Department of Environmental Quality to interpret data from this year alongside historical measurements to fully explain the combination of factors that caused the 2018 algae bloom.

The Gallatin River Task Force has developed a webpage with more information on algae blooms and local river conditions that will be updated as more information becomes available.